Insulin-dependent diabetes mellitus (IDDM; type I diabetes) is one of the most commonly occurring metabolic disorders in the world. In the United States, IDDM affects approximately one in 300 to 400 people, and epidemiological studies suggest that its incidence is continuing to increase. IDDM is caused by an autoimmune response that results in the T lymphocyte-mediated destruction of the insulin-producing .beta.-cells of the pancreas. Castrano et al., Ann. Rev. Immunol. 8:647-679 (1990). Unfortunately, however, although the general mechanism by which IDDM occurs is known, IDDM becomes clinically evident only after the vast majority (approximately 80% or more) of the pancreatic .beta.-cells have been irrevocably destroyed and the individual becomes dependent upon an exogenous source of insulin. In other words, at the time that the disease becomes clinically evident, the autoimmune response is well established and has already caused irreparable damage to the insulin-producing pancreatic tissue.
Because autoimmune-induced pancreatic damage is far progressed by the time that clinical symptoms of IDDM become evident, successful treatment of the autoimmune response ideally should be initiated well before the patient begins to exhibit overt symptoms of diabetes and requires insulin replacement for his or her own lost capability to produce insulin. In fact, any form of therapy would be expected to be more effective if persons at risk could be identified before the onset of clinical symptoms and the concomitant destruction of pancreatic .beta.-cells. Moreover, techniques useful for tracking progression of the IDDM-associated autoimmune response in patients would allow one to assess the effectiveness of therapies which could be employed after the onset of disease. There is, therefore, a need for quick and reliable techniques for diagnostically identifying persons at risk for developing the clinical symptoms of IDDM and for monitoring the progression of the autoimmune response in those persons.
Prior to the onset of clinical symptoms of IDDM but after the onset of the IDDM-associated autoimmune response, attempts have been made to control the established diverse autoreactive T cell population, thereby effectively inhibiting progression of the disease. For example, immunosuppressants and antibodies which are specifically directed against autoimmune T cells may be useful for delaying the onset of disease. However, such treatments lack specificity and often significantly debilitate immune system function. Moreover, immunotherapeutics directed at blocking T cell-receptor/major histocompatibility complex (MHC) interactions can be highly specific, but may also be confounded by the complexity of the autoreactive T cell population and the genetic diversity of MHCs within the patient population.
Once the clinical symptoms of IDDM become evident, numerous different therapies have been employed for treating the debilitating effects of the disease. For example, by far the most commonly employed therapy for the clinical symptoms of IDDM is exogenous insulin replacement. However, while insulin replacement therapy allows most IDDM patients to lead somewhat normal lives, insulin replacement is also imperfect and does not completely restore metabolic homeostasis. As a result, severe complications including dysfunctions of the eye, kidney, heart, and other organs are common in diabetic patients undergoing insulin replacement therapy.
Another common treatment for the clinical symptoms of IDDM is pancreatic or .beta.-islet cell transplantation. However, the insulin-producing .beta.-cells of transplanted tissues are often rapidly destroyed by the same autoimmune response which had previously destroyed the patient's own pancreatic tissue. Therefore, the use of immunosuppressants after transplantation is common, carrying with it the adverse side effects described above.
As such, in addition to the urgent need for improved methods for the early diagnostic identification of persons who are at risk for developing the clinical symptoms of IDDM and for monitoring the progression of the autoimmune response in those at risk persons, there is also an urgent need for improved methods for therapeutically treating those persons who already exhibit clinical symptoms of the disease. Specifically, there exists a need for methods effective in prolonging the lifetime of tissue transplants, for example pancreatic tissue transplants, in mammals without the use of immunosuppressants and for inhibiting the T cell-mediated autoimmune mechanism underlying the disease.
The autoimmune response underlying IDDM is thought to be mediated by proinflammatory T helper 1(Th1) cells; cells that are known to secrete interferon-.gamma.(IFN-.gamma.) and promote the production of murine IgG2a isotype antibodies that are directed against pancreatic .beta.-cell-associated autoantigens. In contrast to Th1 cells, T helper 2(Th2) cells are known to secrete interleukin-4 (IL-4) and interleukin-5 (IL-5) and promote the production of murine IgG1 isotype antibodies directed against pancreatic .beta.-cell-associated autoantigens. We have herein tested whether at advanced stages of the IDDM-associated autoimmune response, the diverse Th1 cell pool could be downregulated by the induction of an anti-inflammatory Th2response to a single .beta.-cell-associated antigen, thereby effectively reducing the autoimnmune-mediated destruction of the pancreatic tissue.
We have previously shown in an animal model of human IDDM, the nonobese diabetic (NOD) mouse, that a pathogenic Th1 response to the .beta.-cell-associated autoantigen glutamic acid decarboxylase (GAD) arises at 4 weeks of age, concurrent with the onset of insulitis in these animals. Kaufman et al., Nature 366:69-72 (1993). GAD is a mammalian protein which serves to catalyze the rate-limiting step in the synthesis of .gamma.-aminobutyric acid (GABA), a major inhibitory neurotransmitter of the mammalian central nervous system. Spink et al., J. Neurochem. 40:1113-1119 (1983), Huang et al., Proc. Natl. Acad. Sci. U.S.A. 87:8491-8495 (1990), Kobayashi et al., Neurosci. 7:2768-2772 (1987), Chang et al., J. Neurosci. 8:2123-2130 (1988), Bu et al., Proc. Natl. Acad. Sci. U.S.A. 89:2115-2119 (1992), Karlsen et al., Diabetes 41:1355-1359 (1992) and U.S. Pat. No. 5,475,086, issued Dec. 12, 1995. The GAD protein is present on various tissues and exists in multiple isoforms, one of which is GAD65, an antigen found to be associated with pancreatic .beta.-cells. Subsequent to the anti-GAD Th1 response described above, T-cell autoimmunity appears to spread to other .beta.-cell antigens such as a 65 kD heat shock protein (hsp65), insulin B-chain, carboxypeptidase H and peripherin in a cascade of autoimmune responses that ultimately leads to IDDM. Kaufman et al., supra and Tisch et al., Nature 366:72-75 (1993).
Thus, methods for shifting a pathogenic Th1-associated autoimmune response to a protective anti-inflammatory Th2 response would be useful for inhibiting the destructive effects of the autoimmune response and, therefore, for inhibiting and/or delaying the onset of clinical symptoms of IDDM and for prolonging the survival of pancreatic tissue transplants. Moreover, an analysis of the Th1/Th2 profiles of any particular subject will provide information as to risk for developing the clinical symptoms of IDDM and for determining progression of the autoinmmune response and the effect of therapies after the onset of the autoimmune response.